At one time at the City Temple in London, there was in the congregation a restaurateur named Emil Mettler, who was a close friend of Albert Schweitzer and a kind of agent for Schweitzer in Britain.
Mettler would never allow a Christian worker to pay for a meal in his restaurant, but once he did happen to open his cash register in the presence of a Secretary of the London Missionary Society.
The Secretary was astonished to see among the bills and coins a six-inch nail.
What was it doing there? Mettler explained, "I keep this nail with my money to remind me of the price that Christ paid for my salvation and of what I owe Him in return."